The sintered hard metals extensively for use in cutting tools and wear resistant tools are divided broadly into two categories: WC bonded by Co and two different kinds of hard phases bonded by Co. To be sure precise, the latter is a combination by Co of a WC phase and a compound carbide phase constituted by more than one kind of the IVa, Va and VIa group elements, and particularly more than 2 kinds of metals selected from Ti, Ta, Nb and W. The mixed carbide phase has B-1 type crystals, one of an ordinary face-centered cubic structure, and is represented by the general formula of (M.sub.A,M'.sub.B,M".sub.C)(C.sub.u,N.sub.v,O.sub.w).sub.z. Generally, the hard phase having B-1 type crystals (hereinafter referred to as a B-1 type hard phase) has a higher hardness compared with WC and a less chemical reaction with steel. For steel cutting purposes, therefore, is used a sintered hard metal in which a B-1 type hard phase coexists with a WC phase.
The B-1 type hard phase has been called a mixed carbide since it contains only C as a nonmetallic, element, and the production method thereof has heretofore been contemplated on the principle of excluding N and O as impurities. In recent years, however, studies have been made on the effect of addition of N, and active utilization thereof is now in progress. In fact, some of the inventors of the present application have made a proposition in relation to this problem (Laid-Open Gazette No. SHO-51-46508). However, the addition of O has been considered to be improper since firstly CO gas is produced by the reaction thereof to the carbide in the course of sintering thereby permitting porosity to remain in the sintered body, and secondly the control of the total amount of C, N and O is attended by difficulties.